Stark Shift of Rotational Lines in the UV Spectrum of the Charge-Transfer Molecule Benzonitrile

Photophysics of a Fluorescent Non-natural Amino Acid: p-Cyanophenylalanine

The non-natural amino acid p-cyanophenylalanine (Phe(CN)) has recently emerged as a useful fluorescent probe of proteins; however, its photophysical properties have not been systematically examined. Herein, we measure the fluorescence quantum yield and the fluorescence lifetime of Phe(CN) in a series of solvents. It is found that the fluorescence lifetime of Phe(CN) shows a linear dependence on the Kamlet-Taft parameter α of the protic solvents used, indicating that the solute-solvent hydrogen bonding interactions mediate the non-radiative decay rate. Thus, results of this study provide a basis for quantitative application of Phe(CN) fluorescence in protein conformational studies.

Photoinduced Rydberg ionization spectroscopy of the B̃ state of benzonitrile cation

Photoinduced Rydberg ionization (PIRI) spectra of the second excited electronic state of benzonitrile cation were recorded via the origin and 6a1 and 6b1 vibrational levels of the cation ground electronic state. This B<--X transition was verified to be a forbidden 2B2<--2B1 transition with an origin at 17,225 cm-1 above the ground ionic state. By the use of vibronic coupling calculations, as well as symmetry analysis and comparison of the PIRI spectra via different ground vibrational levels, a nearly complete assignment of the vibrational structure was made, and the vibrational frequencies of the B 2B2 state of benzonitrile cation were obtained based on the assignments. Comparisons of the experimental spectra with simulations from the vibronic structure calculations are also used to validate the theoretical procedures used in the simulations.